Field of the Invention
This invention relates to a signal processing circuit for an electrostatic capacity type touch sensor.
Description of the Related Art
The electrostatic capacity type touch sensor has been known as an input device to various kinds of electronic devices such as a mobile phone, a portable audio device, a portable game console, a television and a personal computer.
A conventional electrostatic capacity type touch sensor is explained referring to FIG. 10 and FIG. 11. A touch pad 61 is formed on a PCB (Printed Circuit Board) substrate 60 so that an electrostatic capacitor 62 (capacitance C) is formed between the touch pad 61 and the PCB substrate 60, as shown in FIG. 10. The touch pad 61 is connected to a non-inverting input terminal (+) of a comparator 63 through a wiring 64. A reference voltage Vref is applied to an inverting input terminal (−) of the comparator 63. A constant current power supply 65 is connected to the wiring 64 that connects the touch pad 61 and the non-inverting input terminal (+) of the comparator 63.
Operations of the electrostatic capacity type touch sensor are explained referring to FIG. 11. When a finger 66 of an operator is far away from the touch pad 61, a capacitance associated with the touch pad 61 is C. In this case, a voltage at the touch pad 61 increases from 0 V in a reset state as the electrostatic capacitor 62 is charged with a constant current from the constant current power supply 65. An output voltage of the comparator 63 is inverted when the voltage at the touch pad 61 reaches the reference voltage Vref. A length of time from the reset state to the inversion of the comparator 63 in this case is referred to as t1.
When the finger 66 of the operator approaches the touch pad 61, on the other hand, the capacitance associated with the touch pad 61 increases to C+C′. The increment C′ is a capacitance of a capacitor formed between the finger 66 of the operator and the touch pad 61. As a result, the length of time that the voltage at the touch pad 61 takes from 0 V to the reference voltage Vref increases to t2 (t2>t1). That is, it is possible to detect whether the finger 66 of the operator has touched the touch pad 61 or not, based on a difference (t2−41) in the length of time taken by the transition from the reset state to the inversion of the comparator 63. In other words, the touch pad 61 functions as an ON/OFF switch for data input.
Technologies mentioned above are disclosed in Japanese Patent Application Publication No. 2005-190950, for example.
In the conventional touch sensor, however, there is a problem that the voltage at the touch pad 61 is varied to cause malfunctioning when a noise is applied to the touch pad 61.